texts: Why is the sunset at different times at the same longitude at equinox?

texts:
I happen to live in two places that are literally at the same longitude but several thousand kilometers apart from each other (~47°N and ~68°N). It's just a few days until equinox - and I assumed that the sun would set at the very same time in both places. Equinox is supposed to be at September 22, 13:31 UTC. Using suncalc.org I calculated sunset for the northern location at 19:00h while the sun is supposed to set at 18:55 at the southern location.
Can someone please explain why this is?
I can't think of any effects due to refraction of the light as the angle is the same. The only idea I have that it might be due to the definition of sunset. If it is not calculated using the center of the sun, it might be off as sunsets take much longer in polar regions. Still, I'd be surprised if it was 5 minutes.
Also, I do understand that equinox is at 13:31 UTC which does not necessarily align with the sunset (in fact sunset is 3.5h after equinox). But to make things even more strange, even the next day the sun sets later in the north than it does in the south - I'd expect it the other way round. It is actually another day later, September 24, that the sun will set at exactly the same time in both places - (more than) two days after equinox.
PS: Not an astronomer myself, I'm just curious - please bear with me...


texts: Can it be inferred that our cosmological horizon has increased over time?

texts: If I am right, we can see only those stars that lie within our cosmological horizon, and there may or may not be any stars beyond that. Given last 150 years of using telescopes, and since then our cosmological horizon must have increased by about 150 light years, have we seen any new galaxies, stars? If given enough time, say 500 years, new stars are found, can we be sure they have become visible against not being formed, say, 150 years after big bang?


texts: What would the night sky look like from a planet at the centre of a ring galaxy?

texts: If there were an observer on a planet in the centre of a ring galaxy (omitting the galactic core) what could they expect to see in their night sky? I am currently under the impression that their night sky would be divided by a dense band of stars stretching across the celestial sphere, similar to the milky band of our sky - though to a greater extent. But is there any reason this wouldn't be the case?


texts: Calculating distance using plate scale to measure parallax angle - high school homework question

texts:
I need a reality check. All of the context around this question would take way too long to explain, but I'm perfectly willing if I haven't provided enough information here. I'm trying to figure out if I'm just not getting my daughter's high school astronomy lesson or if the lesson itself is incomplete.

Is the pixel measurement of the distance between the position of a subject across two photos from a moving camera, along with the plate scale of that camera/lens setup, sufficient data to calculate the distance to the subject? Let's pretend for now that conditions were ideal so that the only movement of the camera is pure translation along one axis, no other translation or rotation. Is that sufficient data?

If so, does it require a step to convert from the angle calculated by multiplying that pixel distance by the plate scale to the angle between those perspective points with the subject at the vertex? Or is the angle produced by the calculation from the pixel movement of the object (again, with the camera having produced the movement and the subject fixed) multiplied by the plate scale the correct angle to use in the parallax equation to calculate the distance?

Now suppose, as is the case in this assignment, that the photos we are given to calculate from are quite inconsistent, with probably all cartesian movements occurring between frames, as the apparent size and vertical position of the subject changes as well as the horizontal position. Now can the pixel distance be used to calculate the distance to the subject? If so do you measure diagonally from point to point or can you simply measure the horizontal difference to get a more accurate measurement? It seems to me that even if you can do the calculation from the first example, with ideal reference photos and no other data available, you can't if the photos were not produced in a manner that would isolate other variables.

Maybe I'm overthinking this, but it seems like the assignment is terrible and teaches the wrong lesson. I need a reality check before I consider approaching her teacher about that. Thank you!
Updating to add pics. Everything beyond this was added one day after originally posting the question.
There are three different stages to the lesson. First example with sunset image has a different plate scale than the painting on the wall and the tree, which have the same ~0.15mm/pixel plate scale. She has to do screenshots from a small portion of the browser window. Dumb online learning site is very frustrating. Also it's hard to differentiate between what was originally written and what she has written as she tried to do it before reaching out to me.
The other issue I'm personally having is that there are fundamentally false statements, at least by the standards I have for a science text/course. For instance it says that every camera, no matter how simple or complex, has a fixed plate scale. No, every camera and lens combination does, and only if the lens doesn't zoom. And it seems to me like even changing the focal distance alters the view angle very slightly on at least some of the lenses I've used as a commercial photographer. Another example is the insistence that you are measuring the distance from the camera to the subject. In fact, as the diagram shows, what is being measured is the distance from the subject to the nearest point on the line between the two points where the photos were captured. It's a fine point, but that sort of inaccuracy makes me question everything about the text.
Anyway, thank you for your help!


texts: Given that Kepler's Third Law as stated on most websites doesn't include mass, how does it work for planets in other star systems?

texts:
Kepler's Third Law as stated on websites I found while searching for "how to convert between a planet's orbital period and semi major axis" is $P^2 = A^3$ where P is the orbital period in years and A the semi major axis in AU.

However, why is mass of the central body not included?  If Earth was orbiting at the same distance around a star with more or less mass than our Sun, surely its orbital period would be different?


texts: Will I be able to see the "Beads of Baily" at 99%+ totality?

texts:
Where I am (Vancouver, WA), the Sun will be 99%+ covered. Will I be able to see the "Beads of Baily" when the sun is 99%+ covered in my area ?

Downtown Portland, OR is expected to be 99.4% (according to local news). I expect Vancouver, WA, a suburb just north of downtown Portland, to be over 99% as well (99.3-99.2%??)


texts: The polar circle is the limit for how far north or south the sun will shine 24h. But is there a similar limit for the moon?


texts: Are there stars that don't emit visible light?

texts: Are there stars that do not emit any light in the visible part of the EM spectrum?


texts: Exact ecliptic position of Earth and Moon from Mars as photographed by Curiosity Rover?

texts:
I've been studying the idea of travelling through space and navigation - in particular: Apollo sextant (used by astronauts to plot the position of their spacecraft relative to stars). And, in order to appreciate the fundamental characteristic of the solar system, in terms of planes and orbits, I've also been reading about the route of the recently discovered interstellar Comet 2I/Borisov, which passed through the ecliptic plane from above and exited below the solar system.
The goal is to visualise how planes (planets) might appear from different parts of the solar system (in terms of Line Of Sight) - parallel to the ecliptic plane and from different compass points... I find the eyes.nasa.gov/apps/ website for Mars really useful for moving through and around the solar system.
Finally, considering that the Moon has a 5° tilt to the Earth, and Mars is on the same ecliptic plane as Earth - where exactly was Mars and Earth and the Moon when this photo was taken by Curiosity Rover as detailed on this space.com article?
I guess Mars was positioned near Earth September Equinox, and Earth positioned just after the Perihelion, and full Moon.
I can't quite understand how the moon seems to be much lower than Earth.
EDIT: answering the general position of the Earth and Moon, which is the first step towards determining the exact.
Interestingly, the light seen from Mars is that reflecting from the surface of the Pacific ocean.
On the question of "exact", there is a difference, obviously, between knowing the exact and general position. And the first knowing is most suited for the purposes of science, navigation, research - whereas the second form of knowing (generally) for the sake of natural curiosity. But, I think there is a great deal to be gained from appreciating both the exact and general position (us and the Earth) in space - I think this can broaden our perspective.
This is (in general) the position of the Earth and Moon as seen from Mars...
Firstly, the mountain range in the background is the same as the range visible in the 2nd image below. We can compare and notice the same peak and shape of the summits.

The Dingo Gap is situated at Gale Crater.
Mars, Earth, Moon, in space...
In general... January 31, 2014 - 10:30pm. This broad view looking downwards at the plane.

And again...

And from beneath the plane... showing us just how far ahead Mars was relative to Earth.

Finally, considering the exact position of the Moon in order to answer the question for why the Moon appears to be positioned beneath Earth.
In fact, the Moon, at the time, was midway through its 5° tilt orbit around Earth. And, from certain angles and from such distance from Mars, could have appeared beneath Earth, as shown in the photo taken by Curiosity.

Because our galaxy is at an angle, it is difficult to determine where planets are relative to one another - consequently, from one perspective, as shown in the above image, Earth appears to be ahead of Mars. The only absolute way to determine the general position (visually) of planets is from beneath or above the ecliptic plane(?).


texts: Any theories about universal expansion due to action at the universal event horizon?

texts:
I'm curious if there are any theories about the increasing rate of the expansion of the universe that are centered around gravitational pull from matter outside the event horizon of either the observable universe, or the entire universe?

Since observations point to a uniform rate of expansion in all directions, it would make sense (as most theories speculate) that some force is at work. Most theories invoke dark matter/energy, but is it possible that it is simply gravity?

In my thought experiment I considered this in two scenarios:

Finite and Boundless Universe: There is no limit to the size which the universe can expand, but being finite in size and expanding outward, the accumulation of matter at the very edges of the universe is more dense than that in the observable universe. This dense outer boundary is constantly pulling the rest of the universe towards it. As more matter accumulates at the horizon, gravity increases, and the rate of acceleration expands.
Finite and Bounded Universe: The universe is limited in size, as expanding matter and energy reach the event horizon of this limited universe, it is compacted like a black hole that surrounds the entire universe. As the maximum size is limited in this scenario, matter and energy would infinitely accumulate at the horizon; in turn constantly speeding up the expansion of the universe as all matter is pulled outwards towards the ever-growing horizon.

I'm quite the astronomy nerd, and have a good understanding (as much as a non-professional can) of the physical laws governing the universe on a large scale, so I know there are some immediate problems you could pick apart in the above scenarios, this is just an idea I had to jot down before I forgot. I am interested in whether or not any fleshed out theories similar to this idea exist.


texts: Sunrise and sunset times when compared between the vernal and autumnal equinoxes

texts:
Why is it that sunrise and sunset occur at different times when compared between the vernal and autumnal equinoxes?

Consider Pontianak, Indonesia, 0 deg N, for simplicity. Timeanddate.com gives sunrise and sunset times 5:46 AM, 5:53 PM at 3/21 (vernal equinox) and 5:31 AM, 5:38 PM at 9/23 (autumnal equinox). Given the symmetry of the problem, I would expect that sunrise would occur at the exact same time during the two events. Same for sunset. What is the reason for the 15 minute difference between the time sunrise occurs on the vernal equinox and the time sunrise occurs on the autumnal equinox?


texts: I was interested in tracking the orbit of 1986 DA. I couldn't find any resource when I looked. Just for fun, so any help appreciated.


texts: What is the meaning of grain opacity and why does it affect the formation time of gas giants?

texts: While doing research for my presentation on the formation of gas giants, more specifically the "core-accretion model", I have been stumbling across the term "grain opacity" and don't quite understand it's meaning. From what I've already read, it's a quantity that heavily influences the time it takes for giant gas planets to form and that a lower grain opactiy leads to shorter formation times. But why does the opacity of dust grains in the protoplanetary disk have such an effect? Is it because of the protoplanet cooling off faster?


texts: Why were solar constant measurements before TSIS-1 all about 0.3% high?

texts:
Phys.org's Solar energy tracker powers down after 17 years says:

"The big surprise with TSI was that the amount of irradiance it measured was 4.6 watts per square meter less than what was expected," said Tom Woods, SORCE's principal investigator and senior research associate at the University of Colorado's Laboratory for Atmospheric and Space Physics (LASP) in Boulder, Colorado. "That started a whole scientific discussion and the development of a new calibration laboratory for TSI instruments. It turned out that the TIM was correct, and all the past irradiance measurements were erroneously high."
"It's not often in climate studies that you make a quantum leap in measurement capability, but the tenfold improvement in accuracy by the SORCE / TIM was exactly that," said Greg Kopp, TIM instrument scientist for SORCE and TSIS at LASP.

Question: What was it about either the past measurements or their analysis or calibration that made their measurements of the Sun's output about 0.3% high? What TSI's improvement primarily instrumental, or due to better calibration?


texts: What's the point with Hawking Radiation?

texts:
I saw that Black Holes can evaporate because of Hawking radiation.
Because pair of particle, antiparticle can spawn at any point of the space, the antiparticle could be absorbed by the black hole and the particle go outside of it, like if the black hole where "lightning".
But, if one element of the pair can be absorbed by the black hole, isn't there an equal probability of that the particle will be absorbed, too?
I could imaginate that the probability of fluctuation of going into 0 with an equal probability of +1 or -1 mass over time is nonnull, also that's why black hole could evaporate, or something like it?


texts: How fast is a comet moving when it crosses Earth's orbit?

texts: Is it about the same as Earth's orbital speed?


texts: What (actually) defines an Aten-class near-earth asteroid?

texts:
I just read in the EarthSky.org article Hours after discovery, asteroid swept by that the near earth asteroid 2016 QA2 recently passed the earth at about 85,000 km. The article says that it is a member of the Aten group of asteroids. That article links to this very cool GIF http://www.virtualtelescope.net/2016qa2_28aug2016.gif, however it's 40MB so it takes a while to load.

Wikipedia says:

  Aten asteroids are defined by having a semi-major axis of less than one astronomical unit (AU), the distance from the Earth to the Sun. They also have an aphelion (furthest distance from the Sun) greater than 0.983 AU${}^2$.

${}^2$http://neo.jpl.nasa.gov/neo/groups.html

That link shows the following table and drawings. The text says:

  NEAs are divided into groups (Aten, Apollo, Amor) according to their perihelion distance (q), aphelion distance (Q) and their semi-major axes (a).

Are $q$ and $Q$ really perihelion and aphelion respectively? The table says Aten asteroids have $Q>0.983$ but in the drawing for Aten asteroids it says $Aphelion<1.0167$. I noticed these values could be reciprocals, so there may be something more going on here. But at least on the surface there seems to be some contradiction.

Could someone confirm these definitions with an external source - and help me understand the basis for these limits that deviate from 1 AU by only  +/- 1.7%?

The image below is from this explanation: Near Earth Asteroid / NEO Classifications Based on Locations. It might be helpful for discussion.


texts: So I was out with my friends at a bonfire and we decided to sit down and star gaze, my other friend pointed out a star that looked to be moving and we all followed it with our eyes and t was smaller than the other stars and would disappear for a second and then come back but not blinking. And once it went out of view another one would appear again and just go opposite directions in kind of a circular path not a straight one and they were really fast too. This was like by the Big Dipper btw super confused is there like something that moved in our space that does that, a satellite would’ve gone in a non crazy direction right? And not so many of them..


texts: Why do Uranus and Neptune have more methane than Jupiter and Saturn?

texts:
So the standard theory of the solar nebula is that in the region of the gas planets, ice and rock could condense to form planetesimals, which could then accrete hydrogen and helium to form the gas giants. The giant planets are all mostly hydrogen and helium, but Uranus and Neptune have relatively large amounts of hydrogen compounds like methane (that's what gives them their color).
My question is: why did that happen? How did Uranus and Neptune get their methane? My impression is that all the gas giants were far enough out for methane to condense into ice, so how did Uranus and Neptune end up preferentially with methane?


texts: Why aren't there galaxy-sized balls of iron out there?

texts: Since we know that stars burn out, collapse, or blow up when too many of their atoms fuse into iron, then how come, with all the ancient galaxies we've been observing, there isn't one galaxy whose stars have become giant balls of iron, which would eventually collide into one galaxy-sized iron globule in space?


texts: Why do zodiac constellations not look like their names?

texts:
I am trying to answer the question of one of my youngest students (who is not yet in school), and although going through books, wikipedia etc. I do not have a good answer to his question yet:

Why does none of the 12 zodiac constellations look their respective name?

Joe Rao: Why Don’t Constellations Look Anything Like What They’re Named? offers the following explanation which does not convince my student:

Remember, of course, that a few thousand years ago there was not much to entertain people once the Sun went down. They would go outside under the cover of darkness, look up at the stars, and weave stories or tales about people, animals, and mythical beasts, and use the patterns of stars as illustrations.

The best answer I have so far:

The zodiac names are old, and the constellations have been looking back then more like their actual names.

I am not sure how accurate that answer is and whether there is any better explanation, that's why I am asking the simple question here.
References

History of Zodiac constellations (Wikipedia)
https://www.lpi.usra.edu/education/skytellers/constellations/


texts: Are Tharsis Montes and Hellas Basin a result of the same event?

texts:
Looking at Mars map:

the highest, biggest mountains and their associated highlands are essentially antipodes of the deepest, huge basin. Roughly 180 degrees away, similar latitude except opposite sides of equator... this immediately brings to mind the idea that an asteroid impacted the area of Hellas Basin, and the shockwave traveled clear across the planet causing a bulge of Tharsis Rise, and causing multiple volcanoes to sprout there.

Is there anything to this impression, or am I entirely wrong?


texts:
Disclaimer: I'm an astronomy noob

You sometimes see in (B?) movies, that they infer dates from sky patterns.
I was asking myself if you can encode a place and date in a mostly meaningful pattern? How would you do it? Would it be possible?

I was looking at 1 and maybe the relation between the visible planets?
(Moon, Mars, Saturn)

Why: I want to make something special for my girlfriend (who likes "stars"), the date is special to us.


texts: Why do they think that WDJ0551+4135 is "snowman shaped" i.e. a contact binary?

texts:
The BBC's Huge 'space snowman' is two merging stars says

Researchers have discovered a huge snowman-shaped star with an atmospheric composition never seen before.
It is more massive than our Sun but only two-thirds the Earth's diameter.
The object is thought to have resulted from the merger of two so-called white dwarf stars that often explode as powerful supernovas.
Dr Mark Hollands, of Warwick University, said the team's discovery could help scientists better understand how this process occurs.

and later

But this star, named WDJ0551+4135, has an atmosphere unusually rich in carbon. Dr Hollands said that initially these observations "didn't make any sense. The only way you can explain it is if it was formed through a merger of two white dwarfs".

I assume this means that it is suspected of being a contact binary
Question: Is WDJ0551+4135 thought to be a contact binary only based on chemistry and evolution arguments? Wouldn't a light curve of this object alone be enough, and wouldn't the period be extremely short making this somewhat easier to do?
update: C|NET's Astronomers find a bizarre, dying star that 'didn't make any sense' suggests that the argument is indeed based on chemistry, and that asteroseismology may be required to back it up. It also links to Nature Astronomy An ultra-massive white dwarf with a mixed hydrogen–carbon atmosphere as a likely merger remnant.


texts: Why did Mercury not appear to transit through the middle of the sun?

texts:
Why did Mercury not appear to transit through the middle of the sun (the equator)? I initially thought that this was because of the ecliptic, but, then I thought that actually we'd see it above the solar equator, wouldn't we? So I'm confused.

This video of the transit from NASA shows exactly what I'm talking about. Mercury is shown relatively low on the solar disk.

I'm looking specifically for why it was visible in the bottom half of the sun, not the top half?


texts: Does this photo show the "Little Dipper" and "Big Dipper"?

texts:
There are two clusters of stars that I always thought were the Big Dipper and Little Dipper. But after looking at images of the Little Dipper and Big Dipper online, I am not too sure if that's what they are. Also, I found out that the front edge of the Big Dipper is supposed to be aligned with Polaris (or the North Star), which is part of the Little Dipper, but that's not the case with these clusters of stars that I see.
So last night I took this photo:

I circled the clusters of stars that I thought were the Little Dipper and Big Dipper. I would like to know if either of these clusters are indeed the Little Dipper or Big Dipper. If either of them are not the Little Dipper or Big Dipper, what are they? And where should I look in relation to these two clusters to find the Little Dipper and Big Dipper?


texts: Change in axis of the Earth if the Moon would dissappear

texts:
I read that, if the moon would not be there or if it would dissapear or be destroyed it could cause the earth to rotate into a possible axis of 45° degrees. Because it would be missing the gravitational forces of the moon that keeps te earth in the current axis of +-23.5°. If that happend a part of the earth, for example Europe, could turn in a way that it would be permanently "night".
If I understood that correctly and it is possible, would that transformation take long? Or would it be fast? And with fast I am talking about a couple of years or decades and long about thousands or millions of years.
I am working on a story that would involve this and I am trying to see if I can get the logic to work with a real scientific explanation.


texts: Could the rotation of a black hole cause other planets to rotate?

texts:
Because everything rotates so perfectly, and we know that some blackholes rotate and they're the biggest things in the universe, so a silly question comes to mind.

If you remove all black holes from a galaxy:

Would it still rotate?
Would the rotation of the planets inside it slow down until there's no more rotation/gravity?

I imagine that the black hole has something to do with the fact that we have gravity since it's always rotating and pulling spacetime fabric inward like a tornado. There are 100 million black holes in our galaxy so I think of them as rotation generators.

Last year I read a research paper that the rotation of a black hole can't cause gravity, but forgot why.


texts:
The BBC News article Telescope tracks 35 million galaxies in Dark Energy hunt says:

The aim of the five-year programme is to shed light on Dark Energy - the mysterious force thought to drive an accelerated expansion of the Universe.
The instrument effectively contains 5,000 mini-telescopes. Each one can image a galaxy every 20 minutes.
In just one year scientists will have surveyed more galaxies than all the other telescopes in the world combined.

I wonder if instead that it would be better to call them 5,000 mini-spectrometers or 5,000 mini-slits, but I'm not yet sure how this whole thing works.

Inside DESI are 5,000 optical fibres, each acting as a mini-telescope. This enables the instrument to capture light from 5,000 different galaxies simultaneously, precisely to map their distance from Earth, and gauge how much the Universe expanded as this light travelled to Earth.

Question(s)

How exactly will DESI simultaneously capture individual spectra from 5,000 galaxies using optical fibers? Are the fibers used to route the light to a very, very long single slit which is carefully imaged on a 2D detector, or does it do something more complicated?
Considering that the exposure cadence is 20 minutes (per the article), how are all 5,000 fibers quickly repositioned between exposures? 20 minutes is only 1,200 seconds!

update: Wikipedia's Dark Energy Spectroscopic Instrument says:

The Dark Energy Spectroscopic Instrument (DESI) is a new instrument for conducting a spectrographic survey of distant galaxies. Its main components are a focal plane containing 5000 fiber-positioning robots, and a bank of spectrographs which are fed by the fibers.

Wikipedia does not always serve as an authoritative and accurate source, especially for details. Five thousand robots sounds like the name of a Kraftwerk album, and "bank of spectrographs" is less than fully quantitative.

DESI will scan more galaxies in a single year than all the telescopes in the world combined, UCLA/ Lawrence Berkeley National Laboratory

This answer to What is the dimensions of the smallest object detectable by an optical fiber from a specific distance? shows several implementations of multiple optical fibers picking objects from a focal plane and bringing them to a spectrometer. However I don't think any of them come anywhere near to 5,000 separate fibers!

above: GIF from the video (with exciting music) A 2dF night at the Anglo-Australian Telescope


texts: How does the Sky in Google Earth Pro work?

texts:
For those who don't know, Google Earth Pro has a feature where you can see what the sky should look like based on the time and geographical location.

I know Google Sky says that Google Earth uses their information, is there any way that I could access this through an API?

If that isn't possible, do y'all know of any related databases that reveal how the sky would look at night? (Emphasis on how the sky would look here, I know there are things like the Yale Bright Star Catalouge, but it doesn't tell me where the milky way would appear in the sky).


texts: Generate an uniform distribution on the sky between limits

texts:
I have an analog question of this one : Generate an uniform distribution on the sky

I generate an uniform distribution on the sky, but just a patch of it, with $$\alpha_{min} \leq \alpha \leq \alpha_{max}$$ and
$$\delta_{min} \leq \delta \leq \delta_{max}$$

for Right Ascention, I just have to take uniform distribution between $\alpha_{min}$and $ \alpha_{max}$.

For declination it works if I take $$\delta_{random} = \sin^{-1}\left( \mathrm{Uniform}(\sin\delta_{min},\, \sin\delta_{max}) \right)$$

My problem is that I don't know how to demonstrate this formula for the generation of delinations. Any idea how to make the proof of this formula ?

I saw in Generate an uniform distribution on the sky @RobJeffries post which gives an expression of $P(\delta)$ and demonstrates that $\delta = \sin^{-1}(2P-1)$ where $P$ is a random number between 0 and 1, which is compatible to my formula with $\delta_{min} = -\frac{\pi}{2}$ and $ \delta_{max} = +\frac{\pi}{2}$, but I was not able to generalize his demonstration.

Thanks in advance for your help!


texts: Is Mars expected to go through the tail of Comet Siding?

texts:
This question will likely be answered "in retrospect" in about 8 days.  So it's a great opportunity to "lay it out there" and then let reality prove you right/wrong.

On the 19th of October, Comet Siding is expected to pass Mars very closely (1/3 the distance of the moon from Earth). My assumption is that this means its nucleus.  But comet tails can be very long (100s of millions of miles).  For the sake of this questions let's keep it to "obviously observable" tail and coma.

Additionally "going through" may need a definition.  So let's say that if a line drawn from the outer (obviously observable) coma closest to Mars and the end of the (obviously observable) tail were to intersect with-in 20 miles of the surface of Mars, then we would say that Mars has "gone through the tail" of the comet.

So is Mars expected (or even more fun, do YOU expect Mars) to go through the tail of Comet Siding on the 19th of October?

If so, what will be observable from the surface of Mars? Large number of shooting stars?

NASA is preparing to hide its satellites (repositioning them to the "far side" of Mars)  from comet dust traveling at 35 miles/sec.    Is there any idea of how long the encounter will last?


texts: What factors determines the size of a galaxy?

texts: Recently i saw a post comparing sizes of different galaxies with the Milky Way. The largest one being the Ic 1011.What makes this galaxy so large? What factors decide the size of a galaxy?


texts: What are some good books/online resources for astronomy problem sets?

texts:
I am currently studying for the international olympiad for astronomy and astrophysics (the syllabus can be found here: https://www.ioaastrophysics.org/syllabus/). I am currently using Carroll Ostlie's Introduction to Modern Astronomy and Astrophysics to learn the theory.
However I need a good resource for practicing problems. I always finish the problem set at the back of the textbook, however I would like a greater volume of challenging problems to prepare me for the rigor of IOAA. I am aware that I can redo past year papers from other countries or redo some IOAA papers, but I'm saving them for practice before IOAA. So are there any good (preferably topical) resources that I can use with good, challenging problems? Thanks!


texts:
In the Big Bang Theory, it is said that it started with a small speck of matter, yet we have an unimaginable mass of expanding universe which is assumed as immeasurable. If the theory of "Matter cannot be created nor destroyed" than how is it possible for the Big Bang to be considered true?

What is not correct about my (very amateur) understanding?


texts:
What are the orbital parameters of the Sun such as orbit velocity etc in it's orbit around the Solar System's center of mass? Consider the Sun pointlike or alternatively when talkin about the Sun's movement I mean it's center of mass.
Do not tell me that the Sun is stationary because the planets' masses can be neglected. I do no want such oversimplifications.


texts: Could ancient tectonic activity on Mars be due to it having oceans of sulphuric acid?

texts:
Could Mars' ancient tectonic activity have ceased due to its oceans having been composed of sulphuric acid, which warmed up and evaporated due to the Theia event as ice chunks landed on its surface? Strongly conductive liquid sulphuric acid on early Mars' surface could have been responsible for generating its core spin causing tectonic action which then all died off as a result of its oceans evaporating and thus removing the conductivity responsible for the core spin, whereas Earth's oceans didn't evaporate (because they are water) so can phase to ice ages to prevent overheating.
Why else would Mars have evidence of ancient alternately polarised extrusion from now ceased tectonic activity?


texts: Finding Quintuple Planetary Alignments with SkyField

texts:
I am working on a python script that will use the SkyField and SciPy libraries to find quintuple planetary conjunctions and their corresponding constellation location. Specifically I am looking for dates when the 5 visible planets were all in conjunction within the constellation of Aries. This occurrence should be exceptionally rare and I just need something to find if and when it happened in the last 13K years or so...

I found this SkyField solution here to find conjunctions.

I was able to modify the above solution to find quintuple conjunctions for the last 15000 years. At least I think I did. Here is my solution:

import scipy.optimize
from skyfield.api import load, pi, tau, position_from_radec, load_constellation_map

efile = "de431t.bsp"; # ephemeris to use
ts = load.timescale()

print("Loading ephemeris... \nIf this is the first time running this script, a 3.4Gb file will be downloaded.")

# Load Ephemeris
eph = load(efile)

print("\nPlanetary Ephemeris loaded.\n")

# Define planets
earth = eph['earth barycenter']
venus = eph['venus barycenter']
mercury = eph['mercury barycenter']
mars = eph['mars barycenter']
jupiter = eph['jupiter barycenter']
saturn = eph['saturn barycenter']

constellation_at = load_constellation_map()

# Every month from start year
t = ts.utc(-12999, range(12 * 15000))

print("\nCalculation plaetary locations. This may take a while...\n")

# Where in the sky were the Planets on those dates?
e = earth.at(t)

lat, lon, distance = e.observe(venus).ecliptic_latlon()
vl = lon.radians

lat, lon, distance = e.observe(mercury).ecliptic_latlon()
ml = lon.radians

lat, lon, distance = e.observe(mars).ecliptic_latlon()
mal = lon.radians

lat, lon, distance = e.observe(jupiter).ecliptic_latlon()
jl = lon.radians

lat, lon, distance = e.observe(saturn).ecliptic_latlon()
sl = lon.radians

print("Looking for conjunctions...\n")

# When was Mercury conjoined with the other planets?  Compute their difference in
# longitude, wrapping the value into the range [-pi, pi) to avoid
# the discontinuity when one or the other object reaches 360 degrees
# and flips back to 0 degrees.
relative_lon = (vl - ml + pi) % tau - pi
relative_lon2 = (mal - ml + pi) % tau - pi
relative_lon3 = (jl - ml + pi) % tau - pi
relative_lon4 = (sl - ml + pi) % tau - pi

# Find where all planets are within a degrees of one another...
conjunctions = (relative_lon >= 0)[:-1] & (relative_lon < 0)[1:] & (relative_lon2 >= 0)[:-1] & (relative_lon2 < 0)[1:] & (relative_lon3 >= 0)[:-1] & (relative_lon3 < 0)[1:] & (relative_lon4 >= 0)[:-1] & (relative_lon4 < 0)[1:]

# For each month that included a conjunction, ask SciPy exactly when
# the conjunction occurred.

def f(jd):
    "Compute how far away in longitude Venus and Mercury are."
    t = ts.tt(jd=jd)
    e = earth.at(t)
    lat, lon, distance = e.observe(venus).ecliptic_latlon()
    vl = lon.radians
    lat, lon, distance = e.observe(mercury).ecliptic_latlon()
    ml = lon.radians
    relative_lon = (vl - ml + pi) % tau - pi
    return relative_lon

for i in conjunctions.nonzero()[0]:
    t0 = t[i]
    t1 = t[i + 1]
    print("Starting search at", t0.utc_jpl())
    jd_conjunction = scipy.optimize.brentq(f, t[i].tt, t[i+1].tt)
    print("Found conjunction:", ts.tt(jd=jd_conjunction).utc_jpl())
    e = earth.at(ts.tt(jd=jd_conjunction))
    a, b, d = e.observe(venus).radec()
    vr = position_from_radec(a.radians, b.degrees)
    print("In constellation:", constellation_at(vr))
    print()

This seems to be working and the next step is to plug the dates into XePhem and see what they look like.

Can anyone confirm I am doing this right?


texts: Jupiter's moon Io is the most geologically active celestial body in our system. It has the strongest volcanoes and quakes. Of course in practice these quakes wouldn't be as dangerous as on Earth for someone on the surface due to Io's low surface gravity (0.183 g). Let's ignore that however, do we know some of the strongest Ioquakes, what would be their magnitude on the Richter scale?


texts: Specific word for the point of the terrestrial surface "under" a star or other object in the sky

texts: The title says it : I am looking for the word to use for a point, located on Earth surface (maybe an hypothetical perfectly spherical Earth), that has the same coordinates as a specific star, and therefore can be considered to be "under" the star in layman's terms.


texts: What is the density profile within the Sun's photosphere? Which one of these is wrong?

texts:
The Sun's photosphere contains the Sun's surface as defined by opacity = 2/3 point. I'd like to see the profile of mass density from bottom to top of the photosphere. I did a quick search and got confused.

The image below is found on the Wikipedia Photosphere page. If I trace the dotted line labeled "Density" to the photosphere layer and read the density axis at the top, I read something like $8 \text{ to } 3 \times 10^{-7} \text{ g/cm}^{-3}$, which you could call $1 \times 10^{-6} \text{ g/cm}^{-3}$. However, the Sun section of Wikipedia page where this image is shown says:

  The Sun's photosphere has a temperature between 4,500 and 6,000 K (4,230 and 5,730 °C) (with an effective temperature of 5,777 K (5,504 °C)) and a density of about 1×10−6 kg/m3; increasing with depth into the sun.

and links to the solar-center.stanford.edu page The Sun's Vital Statistics for the $1 \times 10^{-6} \text{ kg/m}^{-3}.$ Converting the units, this is only $1 \times 10^{-9} \text{ g/cm}^{-3}.$

Question: Is it possible to clear up this disparity, and to see a plot of the density versus depth from the bottom to the top of the Sun's photosphere, which would likely contain both positive and negative heights above the Sun's surface?

Source: File:Sun Atmosphere Temperature and Density SkyLab.jpg
Original source: SP-402 A New Sun: The Solar Results From Skylab


texts: Is it poossible to perform powered flight on the upper atmospheres of gas giants?

texts:
After reading about Perseverance, whose main challenge I heard was dealing with the relatively thin atmosphere of Mars, I wondered if the thicker atmospheres of gas giants could allow for powered flight similar to Perseverance?
Obviously this would exclude Jupiter and Neptune, the former due to its highly powerful magnetic field and the latter due to its extremely fast winds. For the remaining two gas giants, could anything achieved powered flight there, perhaps in the upper atmospheres so that the magnetic field isn't as strong? I recall reading Uranus for instance has magnetic fields ranging from 0.1 to 1 Gauss which should be somewhat bearable for any unmanned craft
Could such a hypothetical flyer also fly along the direction of the wind so that they're not blown to pieces as well?


texts: Why bother looking for Earth-like planets?

texts: Is it really necessary to look for Earth-like habitable zone planets, when it is likely that the "humans" to reach that planet will be bio-engineered or entirely mechanical by the time we would be exploring "in person?"


texts: Why does Kepler's "Big Picture" of comet 67P look so strange? And what is the significance?

texts:
The observation of comet 67P by the Kepler space telescope has been reported in various science news outlets recently. For example, the image below appeared in the recent NASA news article NASA's Kepler Gets the 'Big Picture' of Comet 67P and the same or similar images can be seen elsewhere. The article says:

From Sept. 7 through Sept. 20, the Kepler spacecraft, operating in its K2 mission, fixed its gaze on comet 67P. From the distant vantage point of Kepler, the spacecraft could observe the comet's core and tail. The long-range global view of Kepler complements the close-in view of the Rosetta spacecraft, providing context for the high-resolution investigation Rosetta performed as it descended closer and closer to the comet.
During the two-week period of study, Kepler took a picture of the comet every 30 minutes. The animation shows a period of 29.5 hours of observation from Sept. 17 through Sept. 18. The comet is seen passing through Kepler's field of view from top right to bottom left, as outlined by the diagonal strip. The white dots represent stars and other regions in space studied during K2's tenth observing campaign.

I don't really understand this image. It is a GIF assembled from several exposures, but in each frame, individual stars appear as tight clusters of dots for some reason, and the comet moves within a bright, narrow band that crosses the entire image in all frames. Why does this looks so strange?
Also, what is the importance of Kepler deviating from it's observations to concentrate on 67P during this period? What is the scientific importance of this series of observations?

above: Image credit: The Open University/C. Snodgrass and SETI Institute/E. Ryan From here.


texts: When did the first annular eclipse happen?

texts:
A few hundreds of millions of years ago, the moon was closer to the Earth than it is today and hence was of a bigger apparent size. This made every solar eclipse either total or partial. Over the course of time, due to tidal forces between the moon and the Earth, the moon slowly drifted far from Earth decreasing its apparent size when viewed from Earth and hence causing annular eclipses to happen.

That would mean there must have been some day in history when Earth witnessed its first ever annular eclipse. Is there an estimate on when this happened?


texts:
This answer to Does anything orbit the Sun faster than Mercury? explains that while Vulcanoid asteroids may have been plentiful in the past, large ones have currently been ruled out though smaller ones less than about 6 km might still be there. It's difficult to observe them because from Earth it requires pointing close to the Sun, and from spacecraft that are much closer (i.e. Mercury and lower) it's really hot and difficult to do.
I think that the current limits are set by analysis of historical STEREO images, space telescopes in orbit at 1 AU designed to look at and around the Sun. A Search for Vulcanoids with the STEREO Heliospheric Imager
My understanding is that during solar system formation there were asteroids all over the place as things formed and then collided, but as some bodies became large large swaths were swept clean and some bands remained.
Have any theories been put forth explaining why the vulcanoid belt seems to be depleted of large asteroids? Is this band, while quite stable, simply not stable enough? Do some think that it simply was never populated by large asteroids to begin with for some reason?
Question: Where have all the Vulcanoids gone? (sung to the tune of Where have all the flowers gone?)


texts: Why does an irregular moon point its longest axis towards its parent planet?

texts: I'm reading the book Ice Worlds of the Solar System. Page 62, it states that an irregular moon should point its longest axis towards its planet. Why is this the more stable configuration?


texts: How well supported are the theories of black-holes containing their own universes?

texts:
The thought crossed kidnapped my mind one night almost a year ago. I was laying down, but I was very interested in thinking about the secrets of a black-hole.

Nat. Geo. - News article

What does the current state of research tell us about the idea that black holes might form universes?

Don't forget to backup your answer!


texts:
A heavy star should look redshifted, due to gravitational time dilation. How is that incorporated into the calculations of distances to the stars, or is it negligible?

How about an entire region of space that is denser or more energetic than our region? Would that not appear further away, than another region that is less dense than our region?

If we were in a region of low density and energy, and the density of space increases as we look further away, is that something that could theoretically explain Einsteins "blunder"? Can one be sure that this is not the case?

If the universe were collapsing, and gravity does indeed propagate. Wouldn't all distant regions seem to be affected more by gravity than the region of the observer, and thus ever more redshifted? (this due to less gravity in "front" of an approaching region of space than "behind" it, relative to us)

I'm curious of how these things are found to be important or unimportant on a cosmic scale.


texts: Why do Jupiter impact events look dark brown?

texts: When a large asteroid impacts a rocky planet, the impact area becomes molten, looking red. However, when an asteroid or other large object impacts a gas giant, like Jupiter, the impact area turns dark brown. Why does this happen?


texts: What is minimal aperture/magnification to recognize phases of Galilean moons?

texts:
Given you take a (non-amateur) refractor telescope, what are aperture and magnification required to recognize phases of the four Galilean moons?

What did Galileo see:
http://www.astro.umontreal.ca/~paulchar/grps/site/images/galileo.4.html


texts: How can we tell that the Milky Way is a spiral galaxy?

texts: From Earth, we can distinguish the type of another galaxy very easily simply by observing the shape, colour, and structure of the galaxy when we image it. But since we are located inside the Milky Way, we cannot get an "outside view". Thus, how can we tell if we are located in either a spiral or an elliptical galaxy?


texts: Is it possible to see Callisto with the naked eye when it's at its greatest elongation from Jupiter?

texts: A quick search shows that Callisto has an apparent magntidue of 5.65, which would make it easily visible under relatively dark skies. Being the farthest Galilean moon, does it ever get far away from Jupiter for it to be discernable to the naked eye?


texts: How close are we to be able to detect and measure gravitational lensing inside the milky way?

texts:
Gravitational lensing takes advantage of statistically uniform distributions of background objects in order to infer a credible map of dark matter concentrations that isn't too noisy. If there are too many background objects and is not clear there is an uniform pattern of distribution that one can compensate numerically, then the noise of the measurement makes the reading essentially worthless

So, I'm curious if there is any research attempt to try to pull this kind of local measurement of lensing in the "backyard". If one takes the Milky Way's galactic rotation curve, and compares it on what's expected given baryonic matter, one would be able to have a radial distribution of expected dark matter in order to compare with the results of lensing

But is that kind of lensing measurement something that is possible today?


texts: Distribution of Stars in Milky Way and globular cluster analogy

texts:
Just a quick (and hopefully simple) question on the distribution of stars in the Milky Way and in globular clusters. I was wondering why it IS NOT possible to describe the distributions of stars in both these systems by analogy to the distribution of air molecules in a room.

Are we talking of the more random motion of air molecules as well as the relatively large number of collisions between air molecules...something which we wouldn't see in a galaxy. I'm assuming also that air molecules do not have the same central gravtational potential...and therefore do not move relative to some fixed point. That is to say, that it is much more random? Any advise on this point would be greatly appreciated.


texts: How far can you see in solar plasma at just under 1 solar radius?

texts:
I was trying to compose an answer to this question here, about touching the surface of a star, and I was going to mention that the popular depiction of the sun's surface as 'basically lava' (as depicted in Sunshine, for example) is wrong.

The usual definition of a star's radius is where the optical depth is 2/3. At this radius, the density of the sun is about 1/10th of air at sea level and can hardly be described as solid or liquid.

I wanted to get a sense of what this diffuse plasma would look like if you were in it and somehow impervious to the heat and radiation. Would it appear as a thin, glowing fog, through which you could still discern distant objects? Or would it appear as opaque as, say, a bonfire?

Rephrasing in a little more technical way, What is the extinction coefficient of solar plasma at ~1 Solar radius in the visible spectrum?
What is the path length at which all light is effectively attenuated?

(Since it varies by wavelength, I mean specifically the region of the visible spectrum where the plasma is at its most transparent).


texts: How to calculate right ascension of Greenwich?

texts: If I know specific time, t0, how can I calculate RA of Greenwich?


texts:
I brought this 130 EQ telescope and set it up following the manual. I checked collimations and seemed alright, the usual 3 clips with the black solid circle in the middle crisscrossed by the spider veins.
I checked the moon and it shows nice and bright with some really nice details. I was able to locate mars and jupiter and its 3 moons. Mars had its orange hue but was very small. Both looked really bright but very tiny. when i increase the magnification i see a white disc with the  spider veins for both. I am using a 20 mm eye piece. Just wondering if that is a right behavior? What can i do to see a bigger image? I have not tried with the other 10 mm eyepiece that comes standard with the telescope. Any advice would be very much appreciated.


texts: Is the surface of TrES-2b actually dark?

texts:
The exoplanet TrES-2b is known as the darkest planet ever found, reflecting <1% of the light that hits it.

What does it mean in respect to the surface? What would we see below the atmosphere of this planet? Is it a "shadow planet" with an atmosphere so thick and dense that it blocks all light that would reach the surface?


texts: How do kepler orbits account for planetary migration?

texts:
This article got me thinking if kepler orbits explain how a planet moves in a solar system how can a planet migrate as explained in the article. At some point don't the kepler orbits must need some other model to explain their positioning?

  "Jupiter would have moved right through the primordial asteroid belt, emptying it and then repopulating it with scrambled material from both the inner and outer solar system as Jupiter then reversed course and headed back towards the outer solar system," DeMeo and Carry write, stressing that further observations should nail down how faithfully this scenario represents reality.


texts: Heat map of portion of the sky visible for a given observatory on Earth?

texts:
Reading Calculating area of visible sky I started wondering which portion of the sky one could observe how long given the 3D coordinates of an observatory, and two points in time.
In other words, I would love to have a formula $f(\lambda,\varphi,h,t_1,t_2, \delta, \alpha)$ for an observatory at $(\lambda, \varphi, h)$ with longitude $\lambda$, latitude $\varphi$, height $h$ above the geoid which shows how many hours between time $t_1$ and $t_2$ which point $(\alpha, \delta)$ of the sky is visible. Here, $\alpha$ is the right ascension and $\delta$ the declination. I am sure this calculation has been done before, and I am simply missing the search terms.
For convencience, let us assume the reference frame to be the usual WGS84. Let's further assume, that our observatory is on a hill on an otherwise flat landscape, so that we that we do not have to worry about skyline or mountains concealing our view to the sky.
References

ESO's object visibility calculator

Related

Throughout the year, are all parts of the night sky visible from any given point on Earth?


texts: Why is there no Proxima 'a' planet? I'm confused about the naming convention

texts:
Why is there no exoplanet around Proxima Centauri designated with the letter 'a'?  There is, famously, a 'b' and now a newly confirmed 'c'....

I have read that exoplanets around a star are designated from 'a' to z or whatever in alphabetical order, starting from the closest to the star OR beginning with the first one discovered, then the second....


texts: Is it possible that the intense gravity of a black hole converts matter into antimatter? Does this mean that matter or energy that goes inside the black hole is converted to antimatter?


texts:
Who first named the planets Mercury, Venus, Mars, Jupiter, and Saturn?

Agnes Clerke's Catholic Encyclopedia article "Astronomy in the Bible" § Planets claims that "Venus and Saturn are the only planets expressedly mentioned in the Old Testament."


texts: How do you go from transit times to Earth-Sun distance?

texts: In the early 18th century, Halley described a method to determine the value of the astronomical unit by observing a transit of Venus from multiple locations on Earth. How do you go from the transit time at two different locations to the distance between the Sun and the Earth? I found many descriptions in rather vague terms of this method, but what is the math behind it?


texts: Can we measure/detect GR light bending during solar eclipse using ordinary equipment?

texts:
Suppose an amateur astronomer finds himself watching Solar Eclipse. He can see the stars around totality. Is Sun's GR effect strong enough for the amateur astronomer to be able to tell that a particular star is appearing a few arc-degrees (or arcminutes, or arcseconds) away from where it should?

If not, what is the minimum requirement to be able to witness GR for yourself during a Solar Eclipse?


texts: Why is the core of a gas giant supported by electron degeneracy pressure instead of nuclear fusion?

texts:
After a Sun-sized protostar forms, its core will become denser over time due to radiation. The core eventually gets dense and hot enough for hydrogen fusion to take place. In the late phases of the star's life, the core will continue fusing hydrogen until it runs out and outer layers begin fusion hydrogen, while dumping helium into the core. The core will get massive enough that it will begin to contract, and it will eventually become electron degenerate due to the high densities.

However, as the core became denser and denser, it reached temperatures at which fusion could take place, but it did not become dense enough for the matter to become electron degenerate, right? Only later in its life, when the star's hydrogen ran out, would it become dense enough for that.

So then why would gas giant cores be electron degenerate, but not become hot enough for nuclear fusion? Shouldn't they start fusing before they can become electron degenerate, as stars do? Am I misunderstanding this concept entirely, or is there more to it than what I described?


texts: If an observer is falling toward a black hole with its face away from singularity then what will he observe after crossing the event horizon? The reason that why I am asking this question because as far as I know for an outside observer, the falling observer appear to freeze at event horizon i.e. time appear to stop for falling observer. So if the falling observer is able to look outward after crossing event horizon then he will be able to see an infinite amount of time which is impossible. So what will the observer see after crossing the event horizon?


texts: How can 16 Psyche be the core of a protoplanet?

texts: The recent approval of a NASA mission to study the asteroid 16 Psyche brought to my attention that Psyche (about 200 km in diameter) is thought to be the exposed iron core left over from a protoplanet (theorized as being about 500 km in diameter). My question is: what is the mechanism by which a body of that size can accrete, differentiate as thoroughly as Psyche apparently did, and then completely lose its mantle? I have a basic understanding of planetary differentiation and I had the impression that the migration of iron and nickel to the core took a long time, and that by that time major impacts capable of shattering large bodies must have been rare.


texts: Why Is it said that if someone is being sucked inside a black hole then he/she may see that time is running slowly?


texts:
The question Lucky imaging with Celestron 14 - is this result reasonable? includes a beautiful example of a focus test using a Bahtinov mask and the image below from Deep Sky Watch's Home Made Bahtinov Focusing Mask & Templates shows how the middle spike moves left-right with respect to the crossing point of the pair of spikes on either side.
Question: But optically speaking how and why does this happen? What is it about the layout of the three gratings in a Bahtinov mask that makes its indicate focus in this way? Why exactly does changing focus produce a lateral shift of one pattern with respect to the other two? What is it about the mask's peculiar design that makes this happen the way it does?
update: This answer to How does a Bahtinov mask work? explains more what happens, but it does not even begin to address Why exactly...
 Source

Star image produced by Bahtinov focusing mask at different focus positions


texts: Why do certain massive stars leave no remnants?

texts:
Mass and metallicity are the two main determinants for a star's fate. This is simple enough. What's more complicated is how exactly these determine the star's fate. For example, you can see in this image I got from Wikipedia's supernova article:

Generally, as metallicity decreases and mass increases, black holes become more likely to form. At a specific mass and metallicity range, however, there are no remnants left behind. From that point, increasing mass and decreasing metallicity allows for black holes again.
How does this work? Why would there ever be an area in which no remnants are left behind? In fact, it seems almost random, since the range lies right between direct black hole ranges.


texts: "Supernova" is the explosion or the resulting celestial body? Is it incorrect to call the explosion "supernova"?

texts:
Sorry for such a newbie question. I searched and read multiples sources about supernova but I'm not completely sure if what is called "supernova" is the explosion, the resulting celestial body or the event as whole (explosion plus resulting celestial body).

Would it be considered as incorrect if for some reason the explosion of a star is called as "supernova"?


texts: How would latitude affect day/night lengths on a planet with no axial tilt?

texts:
How would the length of day and night change with changes in latitude on a planet with no axial tilt?

Would the poles of the planet be in constant sunlight?


texts: How can the Event Horizon Telescope image Sgr A* when it's not visible from all sites at one time?

texts:
I went to https://eventhorizontelescope.org/array and read about the ten sites listed as part of the EHT, I have a mashed-up screen shot of them below.

I made a little script also shown below with approximate coordinates, and saw that Sgr A* is at a declination of about -29.5 degrees (South).  I spun it around the earth, calculated the dot product between each of the sites' position vectors and all normals pointing to Sgr A* as it rotates around the Earth.

I then plotted the elevation angle above the horizon for all ten sites. I was quite surprised!

Sgr A* is never above the horizon for the Greenland telescope!
Sgr A* is never simultaneously visible even to all of the remaining nine telescopes

Have I made a mistake? If not, how do these ten sites combine their data to image Sgr A*?

class Site(object):
    def __init__(self, name, lat, lon, alt):
        self.name = str(name)
        self.lat  = rads * float(lat)
        self.lon  = rads * float(lon)
        self.alt  = km   * float(alt)
        (clat, slat), (clon, slon) = [[f(x) for f in (np.cos, np.sin)] for x in (self.lat, self.lon)]
        self.norm = np.array([clon*clat, slon*clat, slat])

data = (('Northern Extended Millimeter Array', (44.634, 5.908, 2550.)),
        ('IRAM 30 meter telescope', (37.066, -3.393, 2850.)),
        ('The Greenland Telescope now near Thule Air Base', (76.531, -68.703, 10.)),
        ('Combined Array for Research in Millimeter-wave Astronomy CARMA', (37.280, -118.142, 2196.)),
        ('Kitt Peak National Observatory 12 meter Submillimeter Telescope (SMT)', (1.9583, -111.5967,  2096.)),
        ('Mt. Graham International Observatory 12 meter ALMA prototye', (32.701, -109.892, 3191.)),
        ('The Large Millimeter Telescope Alfonso Serrano', (18.985, -97.315, 4600.)),
        ('ALMA', (-22.971, -67.703, 4800.)),
        ('Caltech Submillimeter Observatory', (19.823, -155.476, 4140.)),
        ('South Pole Telescope', (-90.0, 0.0, 2800.)))

# https://eventhorizontelescope.org/array
# https://astronomy.stackexchange.com/questions/26413/math-behind-a-uv-plot-in-interferometry

datadict = dict(data)

import numpy as np
import matplotlib.pyplot as plt
from skyfield.api import Topos, Loader, EarthSatellite
from mpl_toolkits.mplot3d import Axes3D

halfpi, pi, twopi = [f*np.pi for f in (0.5, 1, 2)]
degs, rads = 180/pi, pi/180
km         = 0.001

sites = [Site(a, *b) for a, b in datadict.items()]

Dec = rads * -29.5  # SgrA_star
cDec, sDec = [f(Dec) for f in (np.cos, np.sin)]
th         = twopi * np.linspace(0, 1, 100001)[:-1]
cth, sth   = [f(th) for f in (np.cos, np.sin)]
zth, oth   = np.zeros_like(th), np.ones_like(th)

SgrA_star = np.stack([cDec*cth, cDec*sth, sDec*oth], axis=1)

for site in sites:
    site.elev = np.arcsin(np.dot(SgrA_star, site.norm))

if True:
    for site in sites:
        plt.plot(degs*site.elev)
    plt.plot(zth, '-k', linewidth=2)
    plt.ylim(-90, 90)
    plt.ylabel('elevation (deg)', fontsize=16)
    plt.show()


texts:
I am trying to understand something I read on wikipedia about M31.

Wikipedia says that M31 appears more than six times as wide as the full Moon.

But I remember that when I watched it naked eye it seamed a lot smaller, even considering that I was able to see only the central region. For this reason I also used the software Stellarium to check it and setting the same magnification for the two objects (see pictures), the wikipedia information still seems wrong ... or most likely I am wrong!

Where is that I fail?

"At an apparent magnitude of 3.4, the Andromeda Galaxy is one of the brightest Messier objects,[15] making it visible to the naked eye on moonless nights even when viewed from areas with moderate light pollution. Although it appears more than six times as wide as the full Moon when photographed through a larger telescope, only the brighter central region is visible to the naked eye or when viewed using binoculars or a small telescope"


texts: Can black holes collide with the Sun? In that case what will happen?

texts:
Can a black hole collide with the Sun?

What will happen in that case? Will the Sun survive?


texts: Powerful-enough telescopes to observe the Lunar Roving Vehicle from Earth?

texts:
Apparently, not even the Hubble telescope (which is actually in orbit, not even on Earth) is powerful enough to observe the LVR... will we ever have the technology to observe the LVR from Earth?
(it would be my dream to be able to observe the Apollo relics with a "retail" telescope. I am no expert in optics, so have no idea if for example "computer-enhanced" zooming could work, instead of a giant mirror?)


texts: Why does this large Newtonian telescope's front cover have two or three holes in it?

texts:
The Michael Bernardo video How to use an Equatorial Mount for Beginners shows a large Newtonian telescope on an equatorial mount.
The cover of the telescope's large aperture shows what looks like three holes each with its own removable sub-cover, but I might be misunderstanding what I'm looking at, perhaps the "central object" is just a handle to rotate the cover when the other two holes are open.
But that raises the question "why two holes" rather than just one. I'm pretty sure that these are not used for this kind of thing but it would be great to know how they are used!


texts: How to determine the formula to calculate this position of reflected Sun?

texts:
I created a geometrical simulation of how the Sun is reflected on a wall along the day and along the year, using Geogebra:

https://www.geogebra.org/classic/dckhbc8h
Playing with parameters, I found an oddity: in some specific cases, changing the "season angle", i.e. inclination of su circle w.r.t. horizon, I can see the "sun reflection line" always passing in same point on the wall along whole year:

Overlapped:

Is this a real behaviour or just an error in my simulation?
Does it happens always at same time of the ay? My simulation currently does not show the times along the "sunLine" segment.
How could I build a Geogebra formula to directly draw the "convergence point" of all the sunLine lines along the year?


texts: Can a planet's changing tilt be aligned with the rotation around its star?

texts: What I'm asking is if the tilt of a planet can "follow" the rotation around its star so that, for example, its north pole would always lean towards the star, and thus always be under daylight. The planet could have a tilt similar to earth's 23.5°. And can that planet have a rotation around its axis, allowing for day/night cycles closer to the equator? Hopefully this image will explain what I mean. The tilt (red line) stays the same, but the pole is leaning toward the star throughout the whole year, while the planet rotates around its axis.


texts: Notable differences if all stars would disappear except the Sun

texts: I am not an astronomy expert, but as of today, all other stars than the Sun have little to no effect on life on Earth. So I was thinking, would there be any differences for the Earth at all if all stars would disappear except the Sun?


texts:
I was taught from Bate Mueller and White, that the proper terms for the closest and furthest points and distances from a body in orbit around another unspecified body are "periapsis" and "apoapsis" as depicted here.

However, in comments to this question, it is strongly suggested that the usage of these terms is incorrect.

“Apses” are the points closest to and furthest from a primary body—see
en.wikipedia.org/wiki/Apsis, among many other sources. These points
are called “peri-” and “apo-” with the suffix pertaining to the body
(e.g. “-gee” for Earth, thus “perigee”). The generic word for a
celestial body is ἄστρον

“Periapsis” would mean “the point closest to the point closest to the
body.” Nonsense. The proper word is “periastron”: “periapse” is a
misconstruction.

Is the usage of "periapsis" really incorrect or perhaps just a misnomer?
Should we replace "periapsis" with "periastron"?


texts: As like Earth, does the moon have a day (24 hours). If it has, how many hours are in a moon day?.


texts:
What can be said about 67P/Churyumov-Gerasimenko axial tilt?

Is it fixed (if so, at what value), only changing with precession (at what rate?) or does 67P "tumble" with axial tilt changing over time? (and if so, at what rate?)

I'm interested in estimating the 'polar night' area of the comet, and how it changes over time - but without knowing the rotation properties this is quite hard to do.


texts:
Astronomic observations in geodesy.

It is well understood that sun and star positions may be expressed in Greenwich hour angle and declination. Or Right Ascension Declination. And may be published reckoned to Universal Time (UT), or TT.

An approximation of UT can be obtained by apply a correction (DUT) to UTC. That results in an expression of time described as UT1. DUT time offsets are published and broadcast (WWV shortwave radio) in 0.1" increments.

I'm looking for daily precise (better than 0.1") DUT corrections.

Current broadcast DUT correction is -0.2".

I have retrieved IERS Bulletin B, which has daily values of UT1-UTC in ms. Current value is -21.996 ms. Is it just me? Is -21.996 ms = -0.022"?

Or is there something in the IERS Bulletin B that I'm missing?


texts: Gaia: What is the difference between CCDs used for astrometry, photometry, and spectroscopy?

texts:
My knowledge of CCDs is that these are sensors which collect photoelectrons. That's about it.

What is a difference between CCDs used for astrometry, spectroscopy, and photometry? As an example, each type of CCD is currently used by Gaia.

Recommendations of where else to read about this at a detailed engineering level is appreciated. Thanks.


texts: With all the stars in the universe, why isn't the sky saturated with light?

texts: Space is dark. But if we direct telescopes to seemingly dark portions of sky, we see that it is filled with galaxies. If the universe is theoretically infinite, wouldn't an infinite number of photons be reaching the earth and illuminating the night sky brighter than it appears?


texts:
Jupiter's trojans are located in the L4 and L5 Lagrange points.
These two points are stable, so why don't all the trojans have already merged into small moons?
And since it's not the case, what prevents them to do so?


texts: Do today's professional astronomers use earth rotation angle or sidereal time?

texts: I have read that "earth rotation angle" is the modern replacement for "Greenwich sidereal time". Is it actually the case that modern professional astronomers are using earth rotation angle? Either way, what is the reason for using the one they use?


texts:
All planetary orbits contain 5 unusually stable points. These points are particularly important because they allow man-made satellites to orbit the Sun with a period equal to that of Earth’s. 3 of these points are collinear. Suppose that   is the distance between the centers of mass of Earth and the Sun. Find the distance from Earth’s center of mass to either one of the other stable points in the Earth-Sun system in terms of...

(I'm not looking for a full solution; I just want to know what these points are called)

What are these points it talks about, and what's their mathematical relation to Earth's orbit?

This still isn't homework; I'm just looking for the name of these points.


texts: What will happen to life on Earth when the Andromeda and Milky Way galaxies collide?

texts:
It is said that the Andromeda and Milky Way galaxies are coming close to each other with a speed of approximately 400000 km/hour. They will be together in the next 4 billion years.

What will happen to life on Earth or human beings on Earth?
If we are about to collide in the next 4 billion years then how long before we should take action for interstellar voyage?
Are scientists working on such projects for an interstellar voyage?
Will we be able to get the something like Earth to go away from this galaxy/Earth/ solar system and, considering speed of human instruments/spaceships, how long will it take to go to a safe place?


texts: Why N$_2$ is a non-absorbing species in the spectrum of the Earth?

texts:
The transmission spectrum of the Earth atmosphere is like that (Kaltenegger & Traub 2009):

As you can see, you can find many absorption lines related to some components of Earth's atmosphere: H$_2$O, O$_2$, CO$_2$, O$_3$, CH$_4$... But the nitrogen, N$_2$, is not detected anywhere in the spectrum. Why N$_2$ is not detected, but O$_2$, for example, is?


texts: The expansion of the universe and how stars and galaxies still collide

texts: How can it be that all galaxies and stars are moving away from each other yet they still collide?  From what I have researched it as an open universe (or many open universes) yet the idea of stars and galaxies colliding boggles me.


texts: Units confusion while calculating flux of a star using distance and luminosity, Python

texts:
I am trying to calculate the flux of various stars in Python. I am using the information from a database containing all stars in Hipparcos, Yale Bright Star, and Gliese catalogs (almost 120,000 stars).

This is the database. The original source is here and it is also hosted on GitHub.

The formula I am using to calculate the flux is
$F=L/(4\pi d^2)$ where L is the luminosity and D is the distance.
In the database, the distance is provided in the 'dist' column and the luminosity is provided in the 'lum' column.

This is the code I used to calculate the flux data:

df["Flux"]=df['lum']/(4*np.pi*df['dist']**2)
df['Flux'].head(10)

And this is the output I received:

0     1.587863e-05
1     1.357072e-05
2     1.572559e-04
3     4.137163e-05
4     2.635442e-05
5     8.254667e-07
6     9.653330e-06
7     1.662834e-05
8     2.539475e-05
9     2.540078e-05

My question is:

What is the unit of luminosity provided in the database?
Are the calculated flux values correct in terms of units, or does it require a unit conversion?


texts: How can I calculate the right ascension of an observer on Earth's surface in the TEME frame with python.skyfield?

texts:
I would like to calculate the right ascension of a point on earth as specified above with the Skyfield Python library, and I can not figure out how to do this.
I know that calculating the geocentric position of the respective Topos object at a specific time  results in a Geocentric object that has an attribute .radec; however I am not confident that this delivers the right ascension in the TEME (Earth centered inertial frame.


texts: I am extracting gamma ray spectrum from Fermi-LAT data. My spectrum is a lot higher than the ones on previous studies of the same object. What could cause a spectrum to go too high?


texts:
I have a homework assignment question on accretions discs (essentially an estimation of the number of electron scatterings, but this is just for background).

There are a few parameters, one of them being $L$, which is the linear size of the medium (the medium in this case being an accretion disc around a blackhole)

Now, I have been given the mass of the black hole. Other than that, nothing else which could give me the linear size of the accretion disc.

Could I assume that the linear extent of the accretion disc is perhaps of the order of a few Schwarzschild radii? Which could be calculated from the mass, which is given.

If anyone could shed some light on this I would be very appreciative. I need a nudge in the right direction on this.


texts: I know we can find meteorites from space in Antarctica because snow is white, and we can find them easily on snow. Is it true that meteorites can also be found elsewhere such as beaches, streets or even embedded in a concrete building. It is just very hard to distinguish them from plain rock on earth?


texts: Is Io's orbit or rotation affected by its volcanism?

texts: The rotations of comets and asteroids are affected by outgassing volatilities. Io is very volcanically active. Does this give Io a varying orbit and a slow rotation? Tidal forces should take longer than the effect of a large eruption. Tidal locking means that any volcano alway pushes Io in the same direction (radially, tangential or inclined) accumulating the same kind of orbital change during all of its its active lifetime.


texts:
My question is about the equivalence about having an event horizon and having a singularity.

In one side the implication looks pretty obvious:

A singularity implies having an event horizon and therefore a black hole. Since the mass is compressed in a zero volume space, if you get close enough there will be a point where the escape velocity gets bigger than the speed of light so you will get a black hole by definition.

But what about the opposite? Does having an event horizon imply the existence of a singularity?

Could it be that you have a neutron star massive enough to reach a escape velocity equal to the speed of light but not strong enough to make the matter collapse?

Even if such star can not exist because the strong force collapses before reaching an event horizon, this doesn't mean an equivalence.

It just means that for some specific value of the maximum strong force this is not possible, but image now an imaginary exotic matter that has a way bigger strong force.

For such "science fiction" matter, it would be possible to reach an event horizon without collapsing to a singularity, right?

Or is it really an equivalence between this two concepts, such that no matter how resistant matter is to collapse it will never reach an event horizon?


texts:
Recently, planets outside our galaxy have been discovered.

How would be the convention to naming these planets? As far as I know, it seems that there is no standard for naming exoplanets even in our galaxy.

But if I'm publishing a scientific paper and I wish to name some of these recently discovered planets, what would be some guildelines on doing so?


texts: Can a black hole be torn apart by SMBHs?

texts:
Suppose two SMBHs (supermassive black holes) are brought close (e.g. during a galactic merger).
Suppose that a "normal" black hole drifts into the space between the SMBHs.
Could the gravitational pull of the SMBHs be sufficient to tear the normal black hole to pieces?

If so, will we be able to examine the fragments, free from being obscured by the black hole's event horizon?


texts: what is the percentage of stars with planetary systems?

texts: I'm having hard times understanding the equation of Drake, i can´t find the number of the variable that refers to the stars with planetary systems (fp)


texts: Why is the Moon receding from the Earth due to tides? Is this typical for other moons?

texts:
After reading the Q&A Is the moon moving further away from Earth and closer to the Sun? Why? about the tides transferring energy to the Moon and pushing it from Earth, I have a question:

How is that energy actually being transferred to the Moon? The creation of tides requires energy, so I would expect that this should take energy from the Moon, slow it and cause it to eventually fall back to the Earth. Why isn't that happening?

Finally, if this is the general mechanism, would other moons that orbit around planets with a liquid surface and causing tides, be receding from their parent planets?


texts: How will the Breakthrough Initiatives affect other astronomy observations?

texts:
BACKGROUND

The Breakthrough Initiatives is in the news because of a $100 million private donation for a SETI project. It doesn't seem to be for building any new observatory (but maybe for buying computing power for handling the data for SETI purposes?). It will use two of the greatest radio telescopes in the world, Parks and Green Banks, and optical Automated Planet Finder Telescope at Lick, some part of the time during 10 years.

QUESTIONS

Does this mean a zero sum redistribution of observatory time from other astronomical purposes to SETI?
How does it work when someone can buy the usage of these telescopes? Isn't there a danger in letting popular scifi ideas decide how the great telescopes should be used? Do recruited big names like Hawkins and Drake decide how they should be used anyway??


texts: Why did the big bang produce a low entropy universe?

texts:
In order for the universe to be as it is, containing stars and galaxies, it is a requirement that the early universe was at a relatively low entropy state.

Why did the big bang produce this low entropy universe?


texts: Dust mass-loss rate from a massive star given a set of parameters?

texts:
I've been looking for examples at how mass-loss rates are determined.

I'm studying a circumstellar dust shell ejected from a Wolf-Rayet star. I have some parameters like, expansion velocity of the shell (60km/s), the dust mass of the shell (0.1 M_sun), the radii of the shell (R_in=10000 AU and R_out=60000) and its age (T=26000 yrs).

I was wondering if there's a formula relating those parameters and if they are sufficient to determine the dust mass-loss rate or its dependent on other factors.

Could anyone help?


texts: Standard conditions for a heliacal rising

texts:
I imagine there are several possible conditions for determining the time of a helical rising of a star, the elevation of the star at sunrise, the latitude of the observer, and the azimuth of the star.

Is there any kind of equation (or plot/graphic) that relates these variables and can be used to predict whether a star will be visible? (Ignoring weather, ie assuming some standard atmospheric condition).


texts: I don’t think that black holes are really black as they are the only ones who have the capacity to bend light. At the centre of the black hole the laws of physics are totally defied, black holes appear black as space is black in colour.


texts: Relationship between Color Index and Temperature

texts:
Is there an exact relationship between color indices and temperature?
I have seen some numerical formulas relating B - V color index to temperature, of the form $B - V = a_1 + \frac{a_2}{T} $ .

I am wondering is there any way to derive these formulas or are they completely empirical?


texts:
I am a UI designer working on a sci-fi strategy video game.
I was wondering wether anybody here might be able to help? I am looking for a top down projection of the Milky Way that labels regions of space. This is in order to start building a server map of linked 'matches'.
The space combat sim Elite:Dangerous has a galactic project split into 42 sectors (see attachment) although I know that most of the regions labelled are made up specifically for the game. Obviously I don't want to copy this, so wondered if there was a more scientific version we could base designs on.
Do regional maps of the Milky Way exist in science? Or is it purely a sci-fi thing?


texts: How do sunspots move seen from the side?

texts: Sunspots rise vertically? Do they float on the surface like oil? Do they flow up or down vertically from the Sun's core like a lava lamp?


texts: Did the Late Heavy Bombardment Period Happen Because of a Stellar System Collision?

texts: I was thinking about the impending collision between M31 and the milky way galaxy, and I've heard that since a galaxy is mostly space most of the contents of both galaxies would be just fine. Could a similar effect be responsible for the late heavy bombardment era? Just instead of galaxies, stellar systems?


texts: Does a gravitational wave loses energy over distance?

texts:
How does a gravitational wave travel over vast distances without losing its energy? Could they go on forever as long as the Space?

Thank you


texts: Why does any planet look like a (not so) big white ball through my telescope?

texts: I have a 130mm (5") diameter Newtonian reflector with a 650mm focal length (f/5). I have collimated my telescope already but when I'm looking at Mars or Jupiter (not a star I'm sure), all I see is a white dot. Even if I use my 6mm with a 2x barlow, Mars still looks like a white dot but with a liiittle bit of a gray shade like a 3/4 moon. Of course I focused my eyepiece before. Why can't I see Mars or any other planet better than that ?


texts:
While researching about galaxies in Google sky, I came upon a strange area.
Co-ordinates: 11h 6m 15.0 seconds | -77° 22' 9.8"
A lot of space around this body is empty, is it possible that the body is a black hole?
The reason I'm asking is because of this image -
This is a photograph and readings are of a real black hole..
It looks like the one on the co-ordinates which I observed with all the dust around it.
The co-ordinates of the real black hole are - 03h 19m 47.60s | +41° 30' 37.00"


texts:
I have a celestron powerseeker 114eq telescope. Aperture : 114mm Focal Length: 900mm
I have 20mm and 4mm eyepieces and a 3x Barlow. I have seen Saturn's rings using 4mm eyepiece but it's blurry and with 3x it becomes impossible to see any detail. With 20mm with 3x barlow they are too small. I found a kit on amazon with 6mm and 12.5mm kellener eyepieces with a moon filter. I wanted to view a little larger than 20mm with 3x barlow and with details like color contrasts. Could anyone tell whether the kit that I found will be useful or anything else to suggest to solve my problem?


texts: How to plot celestial equator in galactic coordinates? Why does my plot appear "backwards"?

texts:
I'm trying to plot celestial equator in galactic coordinates. Here is my code (based on this answer):
import matplotlib.pyplot as plt
import seaborn as sns; sns.set()
import numpy as np

from astropy.coordinates import SkyCoord
from astropy import units as u

ra_all = np.linspace(-180, 180, 100)
dec_0 = np.zeros(100)

# Transform equatorial coordinates to galactic
eq = SkyCoord(ra_all, dec_0, unit=u.deg)
gal = eq.galactic
l_plot, b_plot = gal.l.wrap_at('180d').radian, gal.b.radian

lon = np.linspace(0, 360, 100)
lat = np.zeros(100)

# Transform ecliptic coordinates to galactic
ecl = SkyCoord(lon, lat, unit=u.deg, frame='barycentricmeanecliptic')
ecl_gal = ecl.transform_to('galactic')
l_ecl_gal, b_ecl_gal = ecl_gal.l.wrap_at('180d').radian, ecl_gal.b.radian

plt.figure(figsize=(14,7))
plt.subplot(111, projection='aitoff')

plt.scatter(l_plot, b_plot, s=5, label='Celestial Equator')
plt.scatter(l_ecl_gal, b_ecl_gal, s=5, label='Eclptic')

plt.grid(True)
plt.legend(fontsize=16)

plt.show()

However, the map I drew seems to be flipped left to right compared to the one bellow (taken from here)

Question 1: Why does the result of my implementation appear to be the reverse of the other image? Do I have something wrong, or is this simply an issue of mapping the surface from the inside versus from the outside?
Question 2: What is the best way to change my code to make it draw galactic map in a conventional format?


texts: "Table of Astronomy's" depiction of the solar system models

texts:
This is going to be a follow up question to my question about the "Table of Astronomy" from yesterday, What is this “Table of astronomy” about?.

I wanted to take a close look at the three depiction of the solar system models (left to right):

Fig. 43. Ptolemaic System; Fig. 45: Tychonic System; and Fig. 44: Copernican System; They are is this order in the original resource

I cant figure out what some of the symbols and circles are supposed to mean. I have marked everything im unsure about partially or completly in red.

(From left to right again)

Ptolemaic System:

The ptolemaic system is a geocentric system, meaning it has the Earth as its center. I have identified most of the symbols that mark the orbits, although I'm missing two. Furthermore, the 5 outer most orbits(?) are what I'm unsure about. As far as I understand it, Claudius Ptolemy believed not only that the Earth was the center of the universe (making this a model of the universe and not the solar system) but also, that the stars were laying on the inside of the universesphere. Therefore I can imagine that the innermost ring is just to close of the sphere. But there are still 4 rings left.

In addition to the rings, I cant identify the third (anchorlike) and the fifth symbol.

Tychonic System:

Here again I'm unsure about the two outer rings, and the third, not dotted, orbit around the Sun.
The two innermost orbits are Venus and ?

If the (red)dotted orbit is supposed to be the Earth's orbit around the Sun, then why does the earth have two orbits?

Also, what are the two stars under the Moon?

Copernican System:

This one is rather straight forward. I'd like to know which of Saturn's moon's orbits this illustration depicts and if my guesses on Jupiters moons are correct.

And at last, what are the 4 marks, A, B, C and D for?

I hope you can help me! Thanks


texts:
According to Einstein's theory of Special Relativity, one's speed in relation to a relatively stationary object would 'slow down' time on the moving object.

With this in mind, would it be possible to travel in a straight line away from Earth (stationary object) in a space craft able to travel at +90% the speed of light for 2 years and return with significant time dilation?

What technologies have we generated that could reach speeds for significant time dilation (+10 years)?


texts: Creating planispheres from the perspective of nearby objects

texts:
I'm attempting to create a set of [reasonably] astronomically accurate planispheres from the perspective of observers positioned at the north or south poles of various nearby celestial objects. So basically looking out with a 180˚ FOV of each celestial hemisphere.

Using Stellarium and some information I found on this site, I've been able to piece together some decent enough reference images that I should be able to use for this project.

Example: Earth's northern celestial hemisphere

I've been able to make them for objects as far out as Pluto. But... I want to go farther. Let's say Alpha Centauri A.

As far as I know Stellarium doesn't allow you to mount yourself to an object outside of the greater Solar System. This is unfortunate.

I discovered Celestia, which I thought would be the answer. Using Celestia one can fly out to distant stars and wow, what a view!

The problem is that Celestia can't bump the FOV past ~124˚. Manually forcing it to 180˚ causes the program to glitch out.

And here I am. I just can't seem to figure out a way to do this. I've been stuck on this for some time now.

Does anyone know of any software or other tools that may be useful for this project? Celestia has shown me that it's possible. I feel like I'm really close.


texts: What variables are needed to calculate simple horseshoe orbit times?

texts:
EDIT This was NOT a duplicate of Horseshoe orbit cycle times.

But that other question has been deleted, regardless. My original question asked for the answer to the equation(s), and provides many of the variables.  But since no one was apparently willing/able to answer it, this new question asks only for the form of the equation(s) itself and a list of variables involved, so that I can either provide more information on a future question, or try and solve the equations myself (though I doubt my math ability to do that, hence the original question, if I know what equations and variables are involved I can at least make an attempt myself).

What variables need to be known in order to calculate a horseshoe orbit cycle time?

In other words: What is the equation, and what do the variables in that equation represent (speed? mass? semi major axes? etc.), to calculate the time it takes for an object to go from Point A on that image, through Points B, C, D, and E and back to A, along the light blue line in the image above (image also available here)?

For example, Earth and Cruithne complete their cycle in 770 years, while Janus and Epimetheus complete theirs in 8 years.  I'm confident that semi-major axes are key factors in the equation, but I don't know what other variables are included, or how the variables relate to each other in the calculation.

Also, I understand that these orbits are not stable, in the long term, and that my example of Cruithne is especially complex, as it will actually change orbit types periodically, and will likely be ejected from the solar system entirely, or impact the Sun or Jupiter, at some point.  But those complexities are for another time.  For this question, I just want to know the equation in its simplest form.


texts: What are good resources to study Gravitational Waves?

texts: I am undergraduate student having basic knowledge of Physics. I'm looking for resources (books, video lectures, research papers, etc) to study Gravitational Waves from scratch, I think initially I should practice mathematical physics up to some extent and then proceed for GW.


texts:
The smallest observed spiral galaxies I can find are NGC 2976 and NGC 4605, both with a diameter of 20 kly, but I don't have anything like an exhaustive source to search. I've also found a mention that the Small Magellanic Cloud (7 kly diameter) might have had more spiral structure before it was distorted by the Milky Way.
From looking at these galaxies, spiral structure seems to be less clear the smaller the galaxy is, which makes me think there might not be a clear lower limit. Still, I'm curious how small a galaxy with recognizable spiral structure could be, either based on observations or theory.


texts:
I am running MOOG on OS X Yosemite and with $MOOG running the abfind driver I can force fit elemental abundances with an appropriate input line list and model atmosphere. MOOG crashes, however, when I try to use it to generate plots, even though I know I have SuperMONGO (sm) installed on my computer because $sm works fine. In particular, MOOG will crash with the following errors:

*******************************************************************************
                  MOOG IS CONTROLLED BY DRIVER abfind
*******************************************************************************

wav. correl.:  slope =  -7.791E-07  intercept =   7.505  corr. coeff. =  -0.122
Fonts file not found
File graphcap is not defined
Can't get graphcap entry for nodevice
No such device nodevice
File graphcap is not defined
Can't get graphcap entry for x11
No such device x11 -bg black -title MOOGplot -geom 700x800+650+000

How can I get MOOG to find these libraries?


texts: One hour driving gets me Bortle Class 3 light pollution, four hours gets Class 1; how will the darker sky view differ qualitatively?

texts:
I'm still waiting for an answer to Light Pollution: what is the difference between artificial brightness and brightness and SQM but I'm tired of waiting so I'll just go ahead and ask my follow up question now.
I've already seen the night sky at Bortle Class 3, and I could see 7 stars in Ursa Minor which is quite satisfying. But Wiki lists 13 additional stars in that constellation, and if I could see all of them with the naked eye then that would really be something. And if I can only see those stars in Bortle Class 1 then to me it would be worth driving an extra 3 hours.
Question: beyond this abstract argument, is there any way to describe how much better the night sky view is in Bortle 1 than Bortle 3? Beyond a numerical comparison, will there be some qualitative aspects of night sky viewing that will make the experience more worth the drive than a star count?

Background:
I live in San Diego.  If I drive one hour I can get Bortle Class 3 light pollution, if I drive 4 hours I can get Bortle Class 1.  Fortunately, living on the West Coast I have that luxury whereas there really isn't an area east of the Mississippi which has Bortle Class 1.
Anyway, is there a big difference between the two?  I suppose the question is somewhat stupid since the answer is obviously it depends on how badly you want clear skies, but, on the other hand, in music, for example, I really can't tell the difference between notes which are very close to one another.  So if Bortle Class 1 and 3 are more similar to a C and D note of the same octave then the answer is: 'no, don't spend an extra 3 hours in your car', but if it turns out that Bortle Class 1 and 3 are more similar to C notes on adjacent octaves, then, yes, it is worth it.


texts:
I'm new to the forum.
This might be a silly question. I had a confusion about how RA and Dec are calculated. So, from what I understand RA is the angle made by the position vector of an object(position vector as measured with respect to ECI's origin) with vernal equinox direction. Declination is the angle made by the same position vector with the celestial equator(coincides with Earth's equator).
I wanted to make sure that the position vector of the object is considered with respect to ECI's origin. If that is in fact the case, then wouldn't the RA and Dec of a Near-Earth Object depend on the position of the Earth in its orbit ? (If it's a far away star, then the effect of orbital motion of Earth is negligible). Please correct me if I'm fundamentally mistaken in my understanding. Thanks!


texts: What scientific evidence is there to support or refute the Iron Sun Hypothesis?

texts:
Professor Oliver Manuel,  a professor of nuclear chemistry at the University of Missouri-Rolla, claims in the article Sun Is Made Of Iron, Not Hydrogen, Professor Says that

  iron, not hydrogen, is the sun's most abundant element.

Specifically, he and his colleagues postulate that

  "We think that the solar system came from a single star, and the sun formed on a collapsed supernova core," Manuel says. "The inner planets are made mostly of matter produced in the inner part of that star, and the outer planets of material form the outer layers of that star."

Is there evidence to support or refute Professor Manuel's theories?


texts: How can I calculate how the debris of an object ripped apart at the Roche limit will spread out?

texts:
Let's say I have a moon orbiting a planet at a distance $d$. Eventually, $d \leq d_R$, where $d_R$ is the Roche limit of the moon-planet system. I can figure out the mass of both bodies by experiment, as well as their radii, and thus their density (which is how I can calculate the Roche limit).

I'm curious as to how the debris will spread out, though, once the moon is ripped apart. Can I calculate the inner and outer radii of the newly formed ring, based on what I know? Or is there not a direct relationship between these radii and the initial conditions?


texts: What was the age of the universe when the average density was one atmosphere?

texts: How long did it take after the Big Bang for the average density of the universe to reach one Earth atmosphere at sea level? How about the density of water? Is there a chart of densities that relate to everyday objects (so easier to understand)?